1. Field of the Invention
This invention relates to sensors, primarily for sensing an applied force or pressure, which employ an optical fibre which is bent in operation of the sensor so that the light transmittance of the fibre changes and can be detected.
2. Description of the Related Art
Such a use of an optical fibre has been known for several years. In a known example described in U.S. Pat. No. 4572950 and U.S. Pat. No. 4618764, in order to achieve the required bending of the fibre, known as "microbending", in response to an applied force, a filament is wound helically around the fibre and the applied force causes the fibre to be bent into the spaces between the turns of the filament. Winding of the filament complicates manufacture of the sensor, as too does the need for an outer sheath for mechanical protection if the sensor is to be of any practical use. The application of the helical filament and sheath to only a portion of the fibre, if required, is inconvenient. JP-A-60-185626 and JP-A-59-128009 describe sensors which go some way to overcoming the problems associated with the above-mentioned sensor. In these Japanese specifications, the sensor is fitted to a rigid structure, that is to say the window frame of a car door having an electrically-operated window. The sensor has first and second rubber strips or members which sandwich the optical fibre between them. One of the strips has a series of transverse projections which engage the fibre so that when pressure is applied, the other strip bends the fibre into the spaces between the projections of the one strip and thus causes microbending. Because the strips are of a stretchable material, reliance is placed on the strips being attached to the rigid structure of the window frame in order to prevent longitudinal over-stressing of the fibre. This type of sensor is unsuitable as a general purpose sensor which can be placed in any desired position for the detection of pressure. Also, in this type of sensor, there is a risk of damage to the fibre if the fibre is assembled with the rubber strips before one of the strips is attached to the window frame, thus complicating manufacture.